Objective To evaluate the effect of omega-3 polyunsaturaed fatty acid on liver function, donor's T lymphocytes and acute rejection following orthotopic liver transplantation in rats. Methods Orthotopie liver transplantation from BN (RT1 n) to Lewis was performed. Normal saline (NS, n = 6), parenteral nutrition (PN, n = 6) or parenteral nutrition and Omega-3 fatty acid (OM, n = 6) was given through internal jugular vein catheter respectively beginning the first day for consecutive 6 days. At 7thday, the liver function and pathological changes of the transplant liver were observed. The percentage of CD4+ , CD8+, CD4+ CD25+ and CD8+ CD28- T lymphocytes were analyzed by flow-cytometry. Results In OM group the CD4+, CD8+ , CD4+ CD25+ and CD8+ CD28- T lymphocytes significantly decreased, compared with NS (t = 4.28, P < 0.01 ; t = 2.63, P < 0.05 ; t = 2.59, P < 0.05 ; t = 8.86, P <0.01)or PN group(t =8.06, P<0.01; t =3.57, P<0.01; t =5.35, P<0.01; t =7.98, P<0.01). The ratio of CD4+ to CD8+ significantly decreased in OM group compared with NS group and PN group(t = 6.25, P<0.01; t=4.92, P<0.01). RAI (rejection activity index) in NS group, PN group and OM group was 7.17 +0.98, 6.17±0.75 and 4.33±0.52, respectively. The rejection grade in OM group was siguificanfly alleviated than that in NS group and PN group(t =6.25, P <0.01;t =4.92, P <0.01). Conclusion Omega-3 fatty acid inhibits T lymphocytes, especially help T lymphocytes, alleviates acute rejection. The suppressor T cell and the regulatory T cell were not responsible for the alleviation of the rejection.